(A) IFN-γ hypersensitivity and fas priming in progenitors from an FA-C child. All values represent mean clonal growth ± SD. Parallel studies with the same reagents were performed using BM CD34⁺ cells from a normal volunteer (B). Neither the agonistic nor the blocking anti-fas antibodies influence clonal growth of CD34⁺ cells from normal volunteers. IFN-γ (0.05 ng/mL) had no effect (nor has it had an effect in more than 15 separate studies in normal volunteers). As was the case in the FA mice, the fas pathway was primed in progenitors from the child. Specifically, the agonistic antibody suppressed clonal growth and the blocking antibody enhanced clonal growth of both CFU-GM (▪) and BFU-E (□). Moreover, both BFU-E and CFU-GM were suppressed by IFN-γ exposure and the suppressive effect of IFN-γ was blocked completely by the exposure of CD34⁺ cells to the blocking antihuman fas antibody. (C) IRF-1 mRNA is expressed constitutively by BM cells from an FA-C child. CD34⁺ low-density BM cells were exposed to IFN-γ at concentrations of 0.5 ng/mL (lanes 3, 6, and 9) and 50 mg/mL (lanes 4, 7, and 10) for 2 hours (lanes 2, 3, and 4), 6 hours (lanes 5, 6, and 7), and 24 hours (lanes 8, 9, and 10). Lane 1 is a zero time point control (RNA isolated at time zero with no IFN-γ exposure), and lanes 2, 5, and 8 are zero IFN-γ controls at 2, 6, and 24 hours, respectively. The upper set of lanes show IRF-1 amplification from CD34⁻ BM cells from a normal volunteer, and the lower set of lanes show IRF-1 amplification from CD34⁻ BM cells from an FA-C child.